1. Statement of the Technical Field
This invention generally relates to extraction and recovery of water-soluble volatile gas, water vapor and waste heat from stack gas, and more specifically, to such extraction and recovery from the stack gas generated by ovens, such as in the bakery industry.
2. Description of the Related Art
Currently, bakeries and many other industries face two significant economic problems with respect to the use of ovens: (1) low oven efficiency resulting from the loss of waste heat in stack gas and (2) correcting the negative environmental impact of pollutant emissions venting with the stack gas into the atmosphere.
Often, bakeries and other businesses that use ovens are forced to install expensive oxidizing equipment to burn the ethanol and other dangerous pollutant emissions in the stack gas, in order to comply with Environmental Protection Agency (“EPA”) or other governmental regulations. The oxidizers consume additional fuel unrelated to the baking process and generate increased heat losses at the oven site.
The EPA currently requires use of oxidizers as the only technically feasible and suitable control technology for bakery ethanol emissions. Bakeries required to use expensive oxidizing equipment consume additional fuel in order to meet EPA requirements to reduce emissions. In general, the EPA discourages the use of other control devices like carbon adsorption, scrubbers, biofiltration, or condensation. For example, according to the EPA, carbon adsorption units are not recommended on bakery ovens because fats and oils may clog the carbon pores, and ethanol is difficult to strip from the carbon. Also, biofiltration is inefficient because it requires cooling of cool stack gas to an appropriate temperature. In addition, scrubbers are not recommended because of water pollution produced by the scrubber. Finally, the EPA takes the position that condensers are not technically feasible because airflow rates are too high and condensate disposal costs are also too high. See, Alternative Control Technology Document for Bakery Oven Emissions, EPA-453/R-92-017, available online at epa.gov (hereinafter the “EPA Document”), at pages 3-1 to 3-11.
Despite the EPA's preference for oxidizers, condensers and condensation processes have been viewed as promising avenues for recovering ethanol, primarily because of ethanol's ready solubility in water. Some relatively recent patents have focused on condensation principles to remove ethanol from stack gas.
U.S. Pat. No. 5,846,299 to Pravda and U.S. Pat. No. 4,834,841 to Peck both suggest cooling stack gas so as to generate a condensate of water mixed with ethanol. The processes in those patents then increase ethanol concentration and collect the resulting liquid ethanol/water solution for later sale.
In general, the condensation processes involve cooling the stack gas below the dew point of the relevant vapor in the stack gas. Water vapor, at 20 percent by mass in the stack gas, has a dew point temperature of approximately 170° F. As the stack gas further cools to ambient temperature (approximately 70° F.), most of the water vapor in the stack gas will condense out. But ethanol has a lower dewpoint than water vapor and its concentration may be 50 times less. The properties of ethanol are such that it will start to condense at lower temperatures, especially at lower concentrations.
Henry's Law provides that the weight of a gas dissolved by a liquid is proportional to the pressure of the gas upon the liquid. Calculations under Henry's Law show, for example, that ethanol condensation occurs at 28.3° F. for a 1.15 percent concentration and at 2.8° F. for a 0.35 percent concentration. While condensing water vapor could absorb some ethanol as temperature decreases, self condensation of ethanol will not occur in practice above ambient temperature. As an example, under Henry's Law, with the stack gas at 120° F., the amount of water needed to absorb ethanol would be 6 times the amount needed at ambient temperature of 68° F., as shown in FIG. 3.
Peck suggests using the condensation process during cooling of the stack gas, employing a packed column, which increases the surface of the ethanol-absorbing condensate. Using packed beds, Peck increases surface area for ethanol diffusion. However, at the average condensate temperature of 120° F., ethanol's ability to diffuse in water is so low that the amount of condensate would be six times less than would be necessary to completely remove all the ethanol. This assumes that dropwise condensation can even be achieved in an industrial application such as a commercial bakery.
Pravda describes a way to recover ethanol for subsequent sale to end users, suggesting humidifying hot stack gas by adding additional moisture. After addition of the moisture, the stack gas cools, condensing the stack water vapor together with the added water vapor and ethanol. In sum, Pravda's method consumes substantial amounts of clean water in addition to the normal needs of the bakery, thus resulting in significant amounts of hot, dirty condensate, which must then be treated before it is returned to the environment. Moreover, half the condensate would have a temperature above 120° F., leaving it incapable of absorbing any significant amount of ethanol. The recovery of such a small amount of ethanol by using expensive equipment would not seem economically feasible.
In addition to Peck and Pravda, the following references include information of interest, although somewhat less relevant: U.S. Pat. Nos. 635,854; 6,071,116; 5,547,373; 5,544,570; 5,417,198; 5,228,385; 4,492,216; 4,483,243; 4,438,685; and, 3,922,136; U.S. Patent Application Publication Ser. Nos. 2008/0041032 and 2008/0008974; and, foreign patents nos. DE 29602748; DE 3631348; and DE 3314386.